In recent years, environmental problems have been elicited, and reduction in environmental impact has been required in each field.
As the proportion of the amount of carbon dioxide discharged in the field of civil engineering and construction is extremely high based on that in the entire industries, reduction of environmental impact has been eagerly desired particularly in the field of civil engineering and construction.
Most of carbon dioxide discharged in the cement industry is derived from calcination of limestone as a material and burning of fuel used at the time of calcination.
Accordingly, in order to reduce the carbon dioxide emission, it is the most effective method to reduce the amount of calcination of cement clinker, and it is extremely important to promote use of various blended cements having various admixtures mixed therewith with a reduced amount of cement.
However, there has been such a problem that the blended cement is poor in initial strength-development.
The initial strength is deeply relevant to the release cycle and therefore is important in order to shorten the time for construction, and at present, development of a cement composition which provides favorable initial strength-development, particularly strength-development equal to that of ordinary Portland cement even when an admixture is used in an amount exceeding 30%, has been desired.
On the contrary, a slowly cooled blast furnace slag formed as a by-product in a steel plant, is utilized for a roadbed material or a cement material, however, the amount treated is very slight relative to the amount generated, and its effective use has been in search yet.
Accordingly, the present inventors have been conducted extensive studies to overcome the above problems and as a result, they have found that favorable initial strength-development can be obtained by use of a cement composition containing Portland cement having a 3CaO.SiO2 content of at least 60% and a slowly cooled blast furnace slag powder, and particularly, a cement composition using a slowly cooled blast furnace slag powder in an amount of from 10 to 50% provides strength-development equal to that of ordinary Portland cement. The present invention has been accomplished on the basis of these discoveries.